1. Field of the Invention
The present invention relates to a photovoltaic device comprising a silicon-based non-single-crystal semiconductor material.
2. Related Background Art
A photovoltaic device is a semiconductor device for converting optical energy, such as solar light, into electrical energy. As a semiconductor material for such a device, an amorphous material represented by amorphous silicon (a-Si:H) is attracting attention, and is being actively investigated since it is inexpensive, enables a large area formation and a thin film formation, and has a large freedom in the composition, thereby allowing for control of electrical and optical characteristics over a wide range.
In a photovoltaic device comprising the above-mentioned amorphous material, particularly an amorphous solar cell, an improvement in a photoelectric conversion efficiency is an important object.
In order to attain such an object, U.S. Pat. No. 2,949,498 discloses the use of so-called tandem cells, formed by stacking a plurality of solar cells each having a unit device structure. Such tandem cells improve the conversion efficiency by stacking devices of different band gaps and thereby efficiently absorbing different portions of the spectrum of solar light. Tandem cells are so designed that, in comparison with a band gap of a device positioned at a light entrance side (the so-called top layer among the stacked devices), the so-called bottom layer, positioned under such top layer, has a narrower band gap. There is also being investigated a three-layer tandem cell (“triple cell”) having a middle layer between the top layer and the bottom layer.
Also in order to facilitate collection of holes having a shorter diffusion distance among electron-hole pairs generated from an incident light, there is often adopted a configuration in which a p-type layer is positioned at a side of a transparent electrode, namely at a light entering side, thereby increasing an overall light collecting efficiency, and a substantially intrinsic semiconductor (“i-type layer”) is provided between a p-type layer and an n-type layer.
Also, an improvement in a short-circuit current (Jsc) is achieved by employing a microcrystalline silicon in the p-type layer at the light entrance side, utilizing the properties of the microcrystalline silicon having a high conductivity and a small absorption coefficient in a short wavelength region. Also, the microcrystalline silicon, having a wider band gap than in the amorphous silicon, shows a higher efficiency for impurity doping and provides a larger internal electric field in the photovoltaic device. As a result, there are reported improvements in an open-circuit voltage (Voc) and in the photoelectric conversion efficiency (“Enhancement of open circuit voltage in high efficiency amorphous silicon alloy solar cells”, S. Guha, J. Yang, P. Nath and M. Hack: Appl. Phys. Lett., 49 (1986) 218).
However, in such photovoltaic devices, it is difficult to stably control interfacial characteristics of a junction between the p-type layer and the n-type layer, and a change in a junction state or in an amount of impurity causes an increase in a serial resistance and an associated deterioration of I-V (current-voltage) characteristics, thus leading to a fluctuation in the characteristics.